The present invention, in some embodiments thereof, relates to a Brain Computer Interface (BCI) and, more particularly, but not exclusively, to a method and system for classification of an image.
BCI applications depend on decoding brain activity in response to single events (trials), as opposed to delineation of the average response frequently studied in basic research. Electroencephalography (EEG), a noninvasive recording technique, is one of the commonly used systems for monitoring brain activity. EEG data is simultaneously collected from a multitude of channels at a high temporal resolution, yielding high dimensional data matrices for the representation of single trial brain activity. In addition to its unsurpassed temporal resolution, EEG is non-invasive, wearable, and more affordable than other neuroimaging techniques, and is thus a prime choice for any type of practical BCI.
Traditional classification techniques use machine-learning algorithms to classify single-trial spatio-temporal activity matrices based on statistical properties of those matrices. These methods are based on two main components: a feature extraction mechanism for effective dimensionality reduction, and a classification algorithm. Typical classifiers use a sample data to learn a mapping rule by which other test data can be classified into one of two or more categories. Classifiers can be roughly divided to linear and non-linear methods. Non-linear classifiers, such as Neural Networks, Hidden Markov Model and k-nearest neighbor, can approximate a wide range of functions, allowing discrimination of complex data structures. While non-linear classifiers have the potential to capture complex discriminative functions, their complexity can also cause overfitting and carry heavy computational demands, making them less suitable for real-time applications.
Linear classifiers, on the other hand, are less complex and are thus more robust to data overfitting. Linear classifiers perform particularly well on data that can be linearly separated. Fisher Linear discriminant (FLD), linear Support Vector Machine (SVM) and Logistic Regression (LR) are examples of linear classifiers. FLD finds a linear combination of features that maps the data of two classes onto a separable projection axis. The criterion for separation is defined as the ratio of the distance between the classes mean to the variance within the classes. SVM finds a separating hyper-plane that maximizes the margin between the two classes. LR, as its name suggests, projects the data onto a logistic function.
International publication No. WO2014/170897, the contents of which are hereby incorporated by reference, discloses a method for conduction of single trial classification of EEG signals of a human subject generated responsive to a series of images containing target images and non-target images. The method comprises: obtaining the EEG signals in a spatio-temporal representation comprising time points and respective spatial distribution of the EEG signals; classifying the time points independently, using a linear discriminant classifier, to compute spatio-temporal discriminating weights; using the spatio-temporal discriminating weights to amplify the spatio-temporal representation by the spatio-temporal discriminating weights at tempo-spatial points respectively, to create a spatially-weighted representation; using Principal Component Analysis (PCA) on a temporal domain for dimensionality reduction, separately for each spatial channel of the EEG signals, to create a PCA projection; applying the PCA projection to the spatially-weighted representation onto a first plurality of principal components, to create a temporally approximated spatially weighted representation containing for each spatial channel, PCA coefficients for the plurality of principal temporal projections; and classifying the temporally approximated spatially weighted representation, over the number of channels, using the linear discriminant classifier, to yield a binary decisions series indicative of each image of the images series as either belonging to the target image or to the non-target image.